1. Field of the Invention
The present invention relates to a process for producing a composite for use as a positive electrode, based on iron phosphate and in particular LiFePO4, via extrusion in the presence of water or a mixture of water and at least one water-miscible solvent, to the positive electrode obtained by implementing this process and to its applications.
2. Description of Related Art
The invention relates to the field of manufacturing lithium metal polymer (LMP) batteries. This type of battery takes the form of a set of thin films rolled up n times (rolls of the following structure: electrolyte/cathode/collector/cathode/electrolyte/lithium) or of multilayers of n thin films (cut and superposed, or n multilayers of the aforementioned configuration). This unitary stacked/complexed configuration has a thickness of about one hundred microns. It comprises 4 functional sheets: i) a negative electrode (anode) generally consisting of a lithium-metal or lithium-alloy foil, ii) an electrolyte composed of a polymer (generally polyoxyethylene (POE)) and lithium salts, iii) a positive electrode (cathode) composed of an active electrode material based on metal oxide (for example V2O5, LiV3O8, LiCoO2, LiNiO2, LiMn2O4 and LiNi0.5Mn0.5O2 etc.) or based on a phosphate of the LiMPO4 type where M represents a metal cation selected from Fe, Mn, Co, Ni and Ti or combinations of these cations, such as for example LiFePO4, on carbon and on a polymer, and finally iv) a current collector generally consisting of a metal foil and enabling electrical connection.
Processes for producing thin cathode films for lithium batteries generally consist in mixing the active electrode material, which is commonly in powder form, with an electrically conductive material, such as carbon or graphite particles or a mixture of the two, and a polymer binder in an organic solvent to form a homogenous paste. This paste is then applied to a current collector to form a thin film and then the organic solvent is evaporated by heating. The electrode film obtained by these processes is generally porous and contains no electrolyte. This thin cathode film is then joined with the other elements of the battery and then the assembly is saturated with an ionically conductive liquid electrolyte comprising a lithium salt. The porous film forming the cathode is then filled with the electrolyte so as to enable ion exchange between the cathode and the anode.
Other processes for producing thin films of positive-electrode material for solid-state lithium (LMP) batteries employ a mixture incorporating an electrolyte consisting of a solvating polymer and a lithium salt. The mixture comprises the active-electrode material in particle form, the electrically conductive material, the solvating polymer and the lithium salt mixed in an organic solvent to form a homogenous electrode paste. This paste is then applied to a current collector to form a film or thin film, and then the organic solvent is evaporated by heating so as to form the electrode. The positive electrode obtained in this way has a low porosity insofar as the electrolyte is initially introduced into the electrode material before evaporation of the solvent and fills the spaces between the particles of active-electrode material. This positive-electrode film is then joined with a solid ionically conductive separator (polymer electrolyte) and a negative counter electrode in order to form the solid lithium battery.
In both cases, organic solvents are used to reduce the viscosity of the mixture used to manufacture the cathode and allow the electrode paste to be applied to the current collector in the form of a thin film. The organic solvents must then be removed, most often by evaporation after heating, before the various components of the battery are joined together. When this type of electrode is manufactured on an industrial scale or by a continuous process, the evaporated organic solvents must be recovered so as not to pollute the environment. Processes for recovering organic solvents require special facilities to prevent solvent vapor from escaping into the environment, and equipment suited to storing and handling these solvents in large amounts during their use.
Replacing the organic solvents used in these processes with a nonpolluting solvent such as water has already been envisioned, especially in international application WO 2004/045007. According to this process, a support is coated with an aqueous solution containing an active positive-electrode material and a binder consisting of a water-soluble synthetic rubber mixed with a thickening agent. It is then necessary to dry the deposited film on the support for a time of at least 12 to 24 hours so as to reduce the water content down to a value lower than 2000 ppm and preferably lower than 50 ppm. It is not possible to incorporate lithium salts into this solution insofar as these salts, due to their hygroscopic properties, would retain water present in the film and further increase the time required by the drying step to remove the water after coating the aqueous solution on the support. In this case, the film obtained is therefore porous so as to allow it to be subsequently impregnated with a lithium salt during assembly with the other components of the battery and enable ion exchange between the cathode and the anode. The process described in international application WO2004/045007 can therefore not be used to produce lithium-based batteries which require the lithium salt to be incorporated into the positive-electrode material before it is joined with the other components of the battery.
It is also possible to produce positive electrodes by dry (solventless) extrusion. In this case, the various components of the composition of the electrode material are introduced into a single-screw or twin-screw extruder and then extruded through a flat die onto a support. The mixture of the various components of the electrode material however has a high viscosity, thereby generally limiting the content of active electrode material that it is possible to incorporate. Thus, in the case where LiFePO4 is used as active positive-electrode material, the maximum percentage that can be incorporated into the final electrode material is about 65%, more commonly lower than 60% of the total dry weight of the electrode. It is generally not possible, in this respect, to raise the temperature to decrease the viscosity of the system during the extrusion because of the very nature of the polymer used (a polyether), which is sensitive to heat and would be degraded. Moreover, the primary obtained, generally a few hundred microns in thickness, must be rolled or calendered to obtain an electrode film a few tens of microns in thickness, generally ≦65 μm in thickness depending on the applications targeted. This rolling or calendering step cannot generally be carried out directly on the current collector because the compressive and shear stresses related to the viscosity are too great and most often cause the current collector to break (aluminum collector <30 μm in thickness). It is therefore necessary, in a first step, to produce the electrode material and to then continue with an additional step called a complexing step (thermocompression bonding of the cathode to the collector) during which step the material is joined to the current collector. In this context of “stepped” complexing, it is generally more difficult to obtain an optimal quality for the interface between the electrode material and the current collector, whereas in the case of direct rolling or calendering of the electrode material extruded onto the current collector, the rolling or calendering stresses, in addition to their thickness sizing function, strengthen the adhesion of the electrode film to the surface of the collector and thus create a better-quality interface, increasing the homogeneity and quality of electron exchange within the battery.